Building block



FIG. 1 is a front or back view of the building block, showing my new design;

FIG. 2 is a top view of the building block;

FIG. 3 is a right or left side view of the building block;

FIG. 4 is a bottom view of the building block; and,

FIG. 5 is a perspective view of the building block. 

The ornamental design for a building block, as shown and described. 